1. Field of the Invention
The present invention relates to a boride-based substrate for forming semiconducting layers thereon to be used for a semiconductor device, and to a semiconductor device using the boride-based substrate.
2. Prior Art
Gallium nitride based semiconductors have been used for devices for emitting blue to violet rays such as light emission diodes and laser emitting semiconductor devices, and also have been noticed as electron controlling devices having wider energy gaps than silicon and gallium arsenide semiconductors.
In the light emission diodes as examples of conventional semiconductor devices making use of gallium nitride layers, the layers have been formed on the surface for a substrate made of a sapphire crystal, as described in Japanese Patent Publication No. A 4-321280.
FIG. 8 shows a prior art structure of a light emission diode, in which, on a sapphire substrate 20 is formed a GaN buffer layer 11 on which a GaN-grown multilayer is laminated including, in turn, an n-GaN layer 16, an n-AlGaN clad layer 15, an InGaN emission layer 14, a p-AlGaN clad layer 12 and a p-GaN layer 12.
The grown multilayer including p-GaN layer 12 to n-GaN layer 16 is partially cut by etching to expose an upper surface of n-GaN layer 16 on which an n-type electrode 41 is attached, where a p-type unipolar electrode 40 is connected on the top surface of p-GaN layer 12 as a uppermost layer. This type of light emission diode is referred to as lateral type.
Since a single perfect crystal for use of substrates is difficult to be made of GaN, some methods of preparing perfect single crystals containing other substance than GaN have been considered to grow GaN semiconducting layers effectively thereon. For example, proposals have been made to use plane (0001) of spinel MgO.Al2O3 or silicon carbide such as 6Hxe2x80x94 or 4Hxe2x80x94SiC for growing semiconducting layers.
Growth of GaN-based semiconducting layers has usually been achieved by using sapphire crystals. A buffer layer had to be formed previously of AlN or GaN in the amorphous form on the particular plane of the sapphire substrate because no single crystal layer of GaN having few defects can be formed directly on the sapphire crystal due to a considerably large lattice constant differential between an Al2O3 crystal and a GaN crystal.
The lattice of a substrate crystal must match with a GaN or AlN layer to be epitaxially grown thereon to form a perfectly single crystal layer. For the lattice constants of GaN and Al2O3, a reference length, on plane (0001), of sapphire corresponding to the lattice constant of GaN or AlN is 1/1.73 times as long as an a-axis lattice length of a unit lattice cell of sapphire because the GaN or AlN crystal is grown rotating its a-axis by 30xc2x0 to the a-axis of the sapphire unit cell on its plane (0001). The lattice constant a of sapphire is a=4.7580 xc3x85, then the reference length of sapphire being estimated 2.747 xc3x85. On the contrary, GaN and AlN have a lattice constant of 3.168 xc3x85 and 3.186 xc3x85, respectively, then having a percentage of lattice mismatch rate of +15.98% and +13.27%, respectively, which is a ratio of the differential of the nitrides lattice constant from the reference length divided the reference length. The lattice constant of sapphire is excessively larger than that of GaN and AlN from point of view of lattice coherency required for epitaxial growth. This is a reason for preparing a buffer layer on the plane (0001), i.e., the major surface of the substrate, prior to forming GaN crystal layers. Such a buffer layer serves to relax the lattice mismatch between the GaN or AlN layer and the substrate.
However, the GaN grown layers formed on the buffer layer have often included numerous lattice defects, particularly dislocations, which often reach a larger lattice defect density in a range of 107xe2x88x921011 cmxe2x88x921, compared to 102xe2x88x92107 cmxe2x88x921 of the defect density in some GaAs-based semiconducting layer on a GaAs-based semiconducting substrate.
The presence of numerous lattice defects in the GaN grown layers has limited property performance of the produced semiconductor devices and the GaN layers have been required to increase in impurities to be contained therein to emit more carriers. However, this results in further increase in lattice defects created in the semiconducting layers, then deteriorating light emission semiconductor device in all quality performance including parameters such as useful lifetime, withstand voltage, driving voltage, consumable power or power efficiency, operating speed and current leakage.
There was another problem that no electrode was provided for any surface of a sapphire substrate to connect the semiconducting layers in the device to an outer circuit because sapphire is an electrically insulating material having no conductivity across the substrate. By this reason, it was required to provide a unipolar electrode on any conductive or semiconducting layer over a substrate, resulting in limited disposition of electrodes in the device. In the case shown in FIG. 8, an n-type unipolar electrode 41 for connection is attached on a surface of a lower semiconducting layer, e.g., n-GaN layer 16, and accordingly n-GaN layer 16 must previously be provided with a surface by removing partially the layers including p-GaN layer 12 to the n-GaN layer 16 by an etching process. However, thickness of the n-GaN layer 16 whose surface is used for attaching the electrode 41 is not easily controlled to be uniform in the etching process, leading to complicated production processes and low product yield
A sapphire substrate does not efficiently radiate out heat created in the semiconductor device due to its low thermal conductivity, which causes the device to increase in operating temperature, then to decrease in useful life time. This problem was required to be solved by particular measures for promoting heat radiation from the substrate.
There was another problem due to a thermal expansion differential between a sapphire substrate and a GaNxe2x80x94 or AlN-buffer layer, having rather large difference in thermal compression coefficient. In general, the GaN layer gets grown on the surface of a sapphire substrate at a temperature higher than 1000xc2x0 C. During cooling the layer together with the substrate from such a high growth temperature to room temperature, the substrate is contracted so as to form a compression stress in the grown GaN-layer due to a significant difference in thermal compression coefficient between the layer and the substrate, causing residual stress in the GaN layer to increase the density of dislocations and other crystal defects.
An object of the present invention is to provide a substrate for growing directly a semiconducting nitride layer thereon with sufficiently low density of lattice defects in the layer, by approaching the lattice constant of the substrate to that of the nitride layer.
Another object of the present invention is to provide a substrate for growing a semiconducting nitride layer, which substrate has an electric conductivity high enough to form a connecting electrode directly on the front or rear face of the substrate.
Further another object of the present invention is to provide a substrate having high thermal conductivity to effectively radiate heat produced in a semiconductor device formed on the substrate.
Still another object of the present invention is to provide a substrate for growing a semiconducting layer wherein the substrate has a thermal expansion coefficient less different from that of the semiconducting nitride layer to be formed to prevent a residual stress from creating between the substrate and the layer.
Further still another object of the present invention is to provide a semiconductor device using such a substrate which can provide excellent quality performances of high heat radiation and formation of electrodes on the substrate.
In the present invention, a substrate for growing a semiconducting nitride layer comprises a single crystal of a boride having chemical formula XB2 where X is Ti or Zr. The crystal of the boride XB2 has a principal plane (0001), expressed by hexagonal indices, which can be used for a major surface of a substrate on which to grow the layers of a nitride, and has a lattice constant highly coherent in the plane (0001) to that of the nitride, particularly, of gallium Ga, aluminum Al, indium In or boron B.
The present invention includes a semiconductor device comprising the above-mentioned substrate of a boride having chemical formula XB2 where X is Ti or Z and at least one semiconducting layer formed on the substrate, wherein the semiconducting layer includes a nitride semiconductor of a chemical formula of ZN where Z is one of gallium, aluminum and indium and boron.
On the boride XB2 substrate a single crystal layer of the nitride semiconductor can easily be grown epitaxially in very low level of lattice defect density. The plane (0001) of the XB2 substrate has high coherency with respect to the lattice of the single crystalline nitride layer at the interface of both crystals. This reduces the lattice strain in the nitride layer, and then, reduces the amount of dislocations to be induced during growth.
In the present invention, an epitaxially grown nitride layer may be formed directly (i.e., without using any buffer layer) from the substrate surface, and further semiconducting nitride layers may be laminated in turns on the first nitride layer by epitaxial growth steps on the directly grown nitride layer. The direct growth of a semiconducting nitride layer on the substrate can provide advantages of high crystallinity and very low density of lattice defects in the laminated semiconducting nitride layers to improve the above various properties needed for semiconductor devices.
The boride including Ti or Zr in the present invention has a high electric conductivity. It is advantageous that the boride substrate can be used to attach a connecting electrode directly thereon when it is used as a substrate for fabricating a semiconductor device.
The boride including Ti or Zr in the present invention has a relatively high thermal conductivity, then providing another advantage of enhancing the heat radiation created in the semiconductor device from the substrate effectively.
The boride including Ti or Zr has a thermal expansion coefficient close enough to that of the nitrides of Ga or Al to reduce occurrence of residual thermal stress between a boride substrate and a nitride layer, which considerably avoids introducing dislocations into the semiconducting nitride layers during the thermal process, and provides thermally stable performance for the semiconductor device for a long time.